As a liquid metal cooled nuclear reactor or its components are being decommissioned, it is desirable to remove the metal coolant in a manner that is both safe to personnel and that also meets environmental regulations for both radioactive and hazardous wastes. To this end, it is common practice to utilize a two-phase approach of (1) removing the bulk of the metal coolant from the reactor and processing the removed coolant ex-situ in a treatment system that is physically separated from the reactor, and (2) treating the residual metal coolant in-situ in the reactor vessel.
With sodium and NaK metal coolants, treatment must be done with great care, due to both safety and environmental concerns. Sodium and NaK are highly chemically reactive, generating heat and pressure; moreover, their reaction products are typically chemically hazardous and/or flammable in air. Furthermore, their use in the high neutron flux fields of a reactor makes them radioactive and/or causes them to accumulate radioactive fission products. Many treatment approaches have been attempted to address these safety and environmental concerns.
For aqueous treatment of sodium, the chemical reaction at standard temperature and pressure can be written asNa(s)+H2O(l)→NaOH(s)+½H2(g)  (1)
When the reaction is conducted in air, the heat generated by reaction (1) can be sufficient to ignite the hydrogen reaction gas. Sodium hydroxide (NaOH) is a caustic solid that is hazardous to human tissues, including skin and eyes.
For sodium coolant, there are both non-aqueous and aqueous ex-situ treatments. As an example of a non-aqueous treatment, the two-step process of dissolving the sodium in anhydrous ammonia at elevated pressure (15-200 psi) and/or low temperature (−30° C.) to form alkali metal cations and solvated electrons, followed by adding a precipitating agent to combine with the alkali metal cations as taught by Getman in U.S. Pat. No. 6,175,051. Examples of aqueous treatments include the two-step process of injecting the sodium into hot (290° F.) caustic solution and further reacting the sodium hydroxide product with carbon dioxide and evaporating and solidifying the sodium bicarbonate product as taught by Herman in U.S. Pat. No. 5,678,240, and the injecting of sodium into hot caustic solution and then solidifying the caustic product as taught by Lewis in U.S. Pat. No. 4,032,614.
These non-aqueous and aqueous processes are not readily utilized for in-situ treatment of residual sodium, for reasons which include: (1) because the typical design of the nuclear reactor, reactor vessel, and/or its support systems do not support the processing chemistries, physical conditions and/or the equipment configurations required for these treatments, (2) because the two-step processes require at least two separate vessels, and are inherently therefore not suited to being performed in-situ and/or (3) because the process results in a hazardous substance as an intermediate and/or final product, making the process less than would be desired with regards to environmental regulations.
Treating sodium with a concentrated sodium hydroxide solution at room temperature has been found by the inventors to proceed slowly and relatively safely, but with the tendency to build up a white layer of solid material, presumably solid sodium hydroxide, and slow to the degree where the rate becomes undesirably low (as low as 2 mm of sodium depth reacted in 30 days). This slowing of the reaction rate as a consequence of increased concentration of reaction product at the sodium reaction surface is termed “liquid passivation”.
A process previously taught by Sherman in U.S. Pat. No. 6,919,061 discusses reacting sodium with moist carbon dioxide gas to produce a surface layer of sodium bicarbonate, termed “gaseous passivation”. This process is relatively safe but proceeds at a rate inversely proportional to the thickness of the sodium bicarbonate layer (ref. FIG. 7), and as a result the reaction becomes effectively self-limiting and undesirably incomplete in treating residual sodium that exists in thicker deposits.
Treating a sodium residue in-situ and relatively slowly (compared to aqueous treatment) with wet (water) vapor nitrogen and/or with wet (saturated) steam is known to start out proceeding in a relatively safe and controllable manner, but becomes undesirably uncontrollable, due to buildup of sodium hydroxide reaction product interposing at the interface between the sodium residue and the reactant gas. The sodium hydroxide reaction product tends to impede the reaction at the sodium interface, but at the gas interface and with continued treatment its hygroscopic nature attracts and condenses water, which builds up and then reacts suddenly and uncontrollably with underlying sodium when cracks develop in the interposing sodium hydroxide layer.
It is therefore the object of one embodiment of the present invention to provide the heretofore unattained benefits of safe and complete in-situ treatment of one or more alkali metals, without the generation of hazardous products as either intermediate or final products of treatment.